U.S. patent number 6,340,809 [Application Number 09/783,589] was granted by the patent office on 2002-01-22 for gas sensor with ceramic heater.
This patent grant is currently assigned to Denso Corporation. Invention is credited to Hirokazu Yamada.
United States Patent |
6,340,809 |
Yamada |
January 22, 2002 |
Gas sensor with ceramic heater
Abstract
A ceramic heater that may be used in an oxygen sensor for
automotive air-fuel ratio control systems and includes a ceramic
square rod formed with a laminate of a heater substrate on which a
heater-patterned layer consisting of a heater element and leads is
formed and a covering substrate covering the heater-patterned
layer. Metallic terminals are connected electrically to the leads
of the heater-patterned layer, respectively, and mounted on
surfaces of the ceramic square rod opposed to each other in a
direction of lamination of the heater substrate and the covering
substrate, respectively. At least one outer lead is joined to one
of the metallic terminals through a bonding layer.
Inventors: |
Yamada; Hirokazu (Nagoya,
JP) |
Assignee: |
Denso Corporation (Kariya,
JP)
|
Family
ID: |
27277926 |
Appl.
No.: |
09/783,589 |
Filed: |
February 15, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
365173 |
Aug 2, 1999 |
6194693 |
|
|
|
Foreign Application Priority Data
|
|
|
|
|
Aug 3, 1998 [JP] |
|
|
10-219031 |
Jan 14, 1999 [JP] |
|
|
11-8185 |
Apr 27, 1999 [JP] |
|
|
11-120248 |
|
Current U.S.
Class: |
219/543; 219/548;
219/552 |
Current CPC
Class: |
G01N
27/4067 (20130101) |
Current International
Class: |
G01N
27/406 (20060101); H05B 003/16 (); H05B
003/10 () |
Field of
Search: |
;219/260,267,270,520,521,522,523,538,541,542,543,544,548,552,553
;338/314,330,307 ;252/515,518 ;428/446 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Hoang; Tu Ba
Attorney, Agent or Firm: Pillsbury Winthrop LLP
Parent Case Text
The present application claims priority as a divisional application
of U.S. Patent application Ser. No. 09/365,173, filed Aug. 2, 1999,
the entirety of which is incorporated into the present application
by reference.
Claims
What is claimed is:
1. A gas sensor comprising:
a gas sensing element having a gas-exposed portion, said gas
sensing element having formed therein a chamber;
a ceramic heater disposed within the chamber of said gas sensing
element to heat said gas sensing element, said ceramic heater
including,
(a) a ceramic square rod formed with a laminate of a heater element
substrate on which a heater-patterned layer consisting of a heater
element and leads connected to the heater element is formed and a
covering substrate covering the heater-patterned layer of the
heater substrate,
(b) metallic terminals connected electrically to the leads of the
heater-patterned layer of the heater substrate, respectively, said
metallic terminals being mounted on surfaces of said ceramic square
rod opposed to each other in a direction of lamination of the
heater substrate and the covering substrate, respectively, and
(c) at least one outer lead joined to one of said metallic
terminals through a bonding layer;
a first cylindrical holder fitted in the chamber of said gas
sensing element, said first holder including a heater holding
portion for holding said ceramic heater and a sensor contact in
contact with an inner wall of said gas sensing element, the sensor
contact having a sensor signal output terminal;
a second cylindrical holder mounted on an outer wall of said gas
sensing element, having a sensor signal output terminal; and
a slit formed in said first holder to define a C-shaped cross
section, said slit being located substantially
90.degree..+-.20.degree. apart from the sensor signal output
terminal of said first cylindrical holder.
2. A gas sensor as set forth in claim 1, wherein the sensor signal
output terminal of said first cylindrical holder is located
180.degree. apart from the sensor signal output terminal of said
second cylindrical holder.
3. A gas sensor as set forth in claim 1, wherein said slit is
located substantially 90.degree. apart from the sensor signal
output terminal of said first cylindrical holder.
Description
BACKGROUND OF THE INVENTION
1 Technical Field of the Invention
The present invention relates generally to a gas sensor which may
be employed in an air-fuel ratio control system for automotive
vehicles for measuring the concentration of gas such O.sub.2, NOx,
or CO, and more particularly to an improved structure of a ceramic
heater used in gas sensors and a manufacturing method thereof.
2 Background Art
FIGS. 1(a) and 1(b) show one example of conventional ceramic
heaters which is built in an oxygen sensor for use in air-fuel
ratio control of automotive internal combustion engines. The
ceramic heater 9 serves to heat a sensor element up to an elevated
temperature to minimize a variation in measured value.
The ceramic heater 9 consists of a ceramic square rod 10 made of a
laminate of heater substrates and a covering substrate and metallic
terminals 3 mounted on side surfaces 15 of the rod 10. The metallic
terminals 3 connect electrically with leads of a heater-patterned
layer in the rod 10 and joined to outer leads 4 through solders 5,
respectively.
In manufacturing the ceramic heater 9, green sheets 101 and 102, as
shown in FIG. 2(a), whose main component is alumina are first
prepared. Next, a conductive paste is applied to the surface of
each of the green sheets 101 to form a heater-patterned layer 2
consisting of pairs of a heater element 21 and a lead 22. The two
green sheets 101 and the covering green sheet 102 are laid to
overlap each other to form a three-layer laminate. The three-layer
laminate is cut into several pieces as shown in FIG. 2(b). The
metallic terminals 3 are formed on the side surfaces 15 of each
piece which communicate electrically with the leads 22 to make an
intermediate. Subsequently, the intermediate is baked, after which
the outer leads 4 is, as shown in FIG. 2(c), welded to the metallic
terminals 3 through the solder 5. Finally, welded portions of the
outer leads 4 are, as indicated at numeral 6 in FIG. 1(b), plated
with Ni to make the ceramic heater 9.
The above ceramic heater 9 and the manufacturing method thereof,
however, have the following drawbacks.
The metallic terminals 3 are, as described above, mounted on the
side surfaces 15 of the ceramic heater 9. It is, thus, only
possible to attach the metallic terminals 3 to the square rod 10
after the three-layer laminate is cut as shown in FIG. 2(b). In
other words, a large number of terminal attachment processes are
required in mass-production of ceramic heaters.
In addition, the performance of the ceramic heater 9 is usually
inspected after the outer leads 4 are mounted thereon. A large
number of individual inspections are also required in the
mass-production of ceramic heaters, thus resulting in an increase
in manufacturing cost.
Another problem is also encountered in that the ceramic heater 9 is
lower in durability than a round rod heater 91 as shown in FIG.
3(a). The results of heat cycle tests show that portions of the
ceramic heater 9 welded to the outer leads 4 and the metallic
terminals 3 tend to be cracked as compared with the round rod
heater 91. This is because the angle .beta. which each of the
metallic terminals 3 of the ceramic heater 9, as shown in FIG. 4,
makes with the outer surface of the solder 5 is greater than the
angle .alpha. which each of the metallic terminals 3 of the round
rod heater 91, as shown in FIG. 3(b), makes with the outer surface
of the solder 5. The difference between the angles .alpha. and
.beta. depends upon the geometry of the heaters 9 and 91 and thus
is difficult to eliminate. The use of solder which is soft enough
to absorb internal stress ensures substantially the same durability
of the portions of the rod 10 welded to the leads 4 as that of the
round rod heater 91, however, square rod heaters exhibiting higher
durability even in use of harder solder is sought.
SUMMARY OF THE INVENTION
It is therefore a principal object of the present invention to
avoid the disadvantages of the prior art.
It is another object of the present invention to provide an
easy-to-manufacture ceramic heater used in gas sensors which has a
high durability and a manufacturing method thereof.
According to one aspect of the invention, there is provided a
ceramic heater which may be employed in an air-fuel ratio control
system for automotive vehicles for measuring the concentration of
gas such O.sub.2, NOx, or CO. The ceramic heater comprises: (a) a
ceramic square rod formed with a laminate of a heater substrate on
which a heater-patterned layer consisting of a heater element and
leads connected to the heater element is formed and a covering
substrate covering the heater-patterned layer of the heater
substrate; (b) metallic terminals connected electrically to the
leads of the heater-patterned layer of the heater substrate,
respectively, the metallic terminals being mounted on surfaces of
the ceramic square rod opposed to each other in a direction of
lamination of the heater substrate and the covering substrate,
respectively; and (c) at least one outer lead joined to one of the
metallic terminals through a bonding layer.
In the preferred mode of the invention, a second outer lead is
further joined to the other metallic terminal through a bonding
layer.
The metallic terminals are electrically connected to the leads
through holes formed in at least one of the covering substrate and
the heater substrate.
Each of the metallic terminals is mounted on an area inside edges
of the surface of the ceramic square rod.
The bonding layer occupies an area of a surface of the metallic
terminal inside edges of the metallic terminal.
The one of the metallic terminals contains 70 Wt % of W or more.
The bonding layer contains 40 to 98 Wt % of Cu and 2 to 20 Wt % of
Ni.
The bonding layer may contain 60 Wt % of Au or less.
An Ni-plated layer may be formed on the one of the metallic
terminals, having a thickness of 3 .mu.m or less. The outer lead is
joined to the Ni-plated layer through the bonding layer.
According to the second aspect of the invention, there is provided
a ceramic heater. The ceramic heater comprises: (a) a ceramic
square rod formed with a laminate of heater substrates each having
formed thereon a heater-patterned layer consisting a heater element
and first and second leads connected to the heater element and a
covering substrate interposed between the heater substrates; (b)
first and second metallic terminals connected electrically to the
first and second leads of the heater-patterned layers of the heater
substrates, respectively, the metallic terminals being mounted on
surfaces of the ceramic square rod opposed to each other in a
direction of lamination of the heater substrates and the covering
substrate; and (c) outer leads joined to the first and second
metallic terminals through bonding layers, respectively.
In the preferred mode of the invention, the first metallic terminal
is connected to the first leads of the heater substrates through
conductive material-coated holes formed in the covering substrate
and one of the heater substrates. The second metallic terminal is
connected to the second leads of the heater substrates through
conductive material-coated holes formed in the covering substrate
and the other heater substrate.
Each of the bonding layers occupies an area of a surface of one of
the metallic terminals inside edges of the metallic terminal.
Each of the metallic terminals contains 70 Wt % of W or more. Each
of the bonding layers contains 40 to 98 Wt % of Cu and 2 to 20 Wt %
of Ni.
Each of the bonding layers contains 60 Wt % of Au or less.
An Ni-plated layer formed on each of the metallic terminals, having
a thickness of 3 .mu.m or less. The outer leads are joined to the
Ni-plated layers through the bonding layers.
According to the third aspect of the invention, there is provided a
method of manufacturing ceramic heaters which comprises the steps
of: (a) preparing a first green sheet; (b) preparing a second green
sheet; (c) printing a first surface of the second green sheet an
array of heater-patterned layers each consisting of a heater
element and leads connected to the heater element; (d) printing a
second surface of the second green sheet opposite the first surface
with an array of metallic terminals; (e) attaching the first green
sheet to the second green sheet so as to cover the first surface of
the second green sheet to form a laminate; (f) baking the laminate
to form a ceramic board; (g) joining outer leads to the metallic
terminals through bonding layers, respectively; and (h) cutting the
ceramic board into a plurality of square rods constituting units of
the ceramic heaters.
In the preferred mode of the invention, a step is further provided
which forms through holes in the first green sheet for electrical
connections of the leads of the heater-patterned layers and the
metallic terminals.
A step is further provided which forms grooves in a surface of the
ceramic board between adjacent two of the units of the ceramic
heaters to be cut by the cutting step.
According to the fourth aspect of the invention, there is provided
a method of manufacturing ceramic heaters which comprises the steps
of: (a) preparing a first green sheet; (b) preparing second green
sheets; (c) printing a first surface of each of the second green
sheets an array of heater-patterned layers each consisting of a
heater element and leads connected to the heater element; (d)
printing a second surface of each of the second green sheets
opposite the first surface with an array of metallic terminals; (e)
interposing the first green sheet between the second green sheets
so as to cover the first surfaces of the second green sheets to
form a laminate; (f) baking the laminate to form a ceramic board;
(g) joining outer leads to the metallic terminals formed on at
least one of the second green sheets through bonding layers,
respectively; and (h) cutting the ceramic board into a plurality of
square rods constituting units of the ceramic heaters.
According to the fifth aspect of the invention, there is provided a
gas sensor which comprises: (a) a gas sensing element having a
gas-exposed portion, the gas sensing element having formed therein
a chamber; (b) a ceramic heater disposed within the chamber of the
gas sensing element to heat the gas sensing element; (c) a first
cylindrical holder fitted in the chamber of the gas sensing
element, the first holder including a heater holding portion for
holding the ceramic heater and a sensor contact in contact with an
inner wall of the gas sensing element, the sensor contact having a
sensor signal output terminal; (d) a second cylindrical holder
mounted on an outer wall of the gas sensing element, having a
sensor signal output terminal; and (e) a slit formed in the first
holder to define a C-shaped cross section, the slit being located
90.degree..+-.20.degree. apart from the sensor signal output
terminal of the first cylindrical holder. The ceramic heater
includes, (a) a ceramic square rod formed with a laminate of a
heater substrate on which a heater-patterned layer consisting of a
heater element and leads connected to the heater element is formed
and a covering substrate covering the heater-patterned layer of the
heater substrate, (b) metallic terminals connected electrically to
the leads of the heater-patterned layer of the heater substrate,
respectively, the metallic terminals being mounted on surfaces of
the ceramic square rod opposed to each other in a direction of
lamination of the heater substrate and the covering substrate,
respectively, and (c) at least one outer lead joined to one of the
metallic terminals through a bonding layer.
In the preferred mode of the invention, the sensor signal output
terminal of the first cylindrical holder is located 180.degree.
apart from the sensor signal output terminal of the second
cylindrical holder.
The slit is located 90.degree. apart from the sensor signal output
terminal of the first cylindrical holder.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more fully from the
detailed description given hereinbelow and from the accompanying
drawings of the preferred embodiments of the invention, which,
however, should not be taken to limit the invention to the specific
embodiments but are for the purpose of explanation and
understanding only.
In the drawings:
FIG. 1(a) is a perspective view which shows a conventional ceramic
heater;
FIG. 1(b) is a cross sectional view taken along the line A--A in
FIG. 1(a);
FIGS. 2(a), 2(b), and 2(c) are perspective views which show a
sequence of manufacturing processes of a conventional ceramic
heater;
FIG. 3(a) is a perspective view which shows a conventional ceramic
heater made of a round bar;
FIG. 3(b) is a cross sectional view taken along the line B--B in
FIG. 3(a);
FIG. 4 is a sectional view which shows a welded angle of an outer
surface of an end of a bonding layer with a metallic terminal;
FIG. 5(a) is a perspective view which shows a ceramic heater
according to the invention;
FIG. 5(b) is a sectional view taken along the line C--C in FIG.
5(a);
FIG. 6 is an exploded view which shows the ceramic heater in FIG.
5(a);
FIGS. 7(a), 7(b), and 7(c) are perspective views which show a
sequence of manufacturing processes of a ceramic heater;
FIGS. 8(a) and 8(b) show modifications of an outer lead connected
to a ceramic heater;
FIG. 9 is a graph which shows the relation between the hardness of
solder and a component ratio of Au to Cu of the solder;
FIG. 10 shows the second embodiment of the manufacturing processes
of the ceramic heater 1.
FIGS. 11(a) and 11(b) show manners to measure the surface roughness
of a metallic terminal of the invention and a conventional metallic
terminal;
FIG. 12 is a vertical sectional view which shows an oxygen sensor
in which the ceramic heater shown in FIGS. 5(a) and 5(b) is
built;
FIG. 13(a) is a perspective view which shows a minus holder for
holding a gas sensing element;
FIG. 13(b) is a perspective view which shows a plus holder for
holding a ceramic heater;
FIG. 14(a) is a plan view of a plus holder;
FIGS. 14(b) and 14(c) are side views of the plus holder in FIG.
14(a);
FIGS. 15(a) and 15(b) are side views of a plus holder in which a
ceramic heater is fitted;
FIG. 16 is a plan view which shows a plus holder in which a ceramic
heater is fitted;
FIG. 17 is a plan view which shows a plus holder holding therein a
ceramic holder fitted in a gas sensing element and a minus
holder;
FIGS. 18(a) and 18(b) are plan views which a comparative example;
and
FIG. 19 is a sectional view which shows a welded angle of an outer
surface of an end of a bonding layer with a metallic terminal.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like numbers refer to like
parts in several views, particularly to FIG. 5(a) and 5(b), there
is shown a ceramic heater 1 of an oxygen sensor according to the
invention which is employed in automotive air-fuel ratio control
systems to measure an oxygen content in exhaust gasses of an
internal combustion engine. Note that the present invention is not
limited to an oxygen sensor and may alternatively be used with a
variety of gas sensors such as HC, CO, and NOx sensors.
The ceramic heater 1 includes a ceramic square rod 10 which is, as
clearly shown in FIG. 6, made of a laminate of two heater
substrates 11 and a covering substrate 12. Each of the heater
substrate 11 has formed thereon a heater-patterned layer 2
consisting of a heater element 21 and leads 22 connected to the
heater element 21. The covering substrate 12 is interposed between
the heater substrates 11 to cover the heater-patterned layers
2.
The ceramic heater 1 also includes a pair of metallic terminals 3
which are attached to upper and lower surfaces 17 and 18, as viewed
in FIGS. 5(a) and 5(b), of the heater substrates 11 and which are
electrically connected to the leads 22. Outer leads 4 are welded to
the terminals 3 through bonding layers 5, respectively.
The covering substrate 12 and the heater substrates 11, as clearly
shown in FIGS. 5(b) and 6, have conductive material-coated through
holes 71, 72, 73, and 74, respectively, to establish electrical
communication between the heater-patterned layers 2 of the heater
substrates 11 and the metallic terminals 3.
The metallic terminals 3 are, as clearly shown in FIG. 5(b),
disposed on flat portions of the surfaces 17 and 18 of the heater
substrates 11 so that side ends 31 thereof may be located inside
side edges 171 and 181 of the heater substrates 11,
respectively.
The bonding layers 5 are formed with solder made of, for example,
Cu/Si, Cu/Au, or Cu/Ni material and, as can be seen in FIG. 5(b),
formed on flat surfaces of the terminals 3 so that side edges 51
thereof may be located inside the side ends 31 of the terminals
3.
The ceramic square rod 10 has, as shown in FIG. 6, an overall
length L of 54 mm, an overall width W of 2.9 mm, and a thickness T
of 1.6 mm (see FIG. 5(a)). The length C of the heater element 21 of
each of the heater-patterned layers 2 is 9 mm. The length D of each
of the leads 22 is 42 mm.
The leads 22 formed on each of the heater substrates 11 extend in
parallel at an interval F2 of 0.228 mm away from each other. Each
of the leads 22 is disposed at an interval F1 of 0.25 mm away from
the side of the heater substrate 11 and at an interval F3 of 1 mm
away from a rear end of the heater substrate 11.
The through holes 71 to 74 are arrayed with a pitch P1 of 3.6 mm in
a lengthwise direction of the heater substrate 11 and a pitch P2 of
1.4 mm in a widthwise direction of the heater substrate 11 and have
a diameter of 3 mm. The metallic terminals 3 each have a length E1
of 5.5 mm and a width E2 of 2.3 mm.
A sequence of manufacturing processes of the ceramic heater 1 will
be discussed below with reference to FIGS. 7(a), 7(b), and
7(c).
A powdered raw material containing about 92 Wt % of Al.sub.2
O.sub.3 and a total of about 8 Wt % of SiO.sub.2, CaO, and MgO is
first prepared to make slurry.
Next, a green sheet is formed with the slurry using the doctor
blade and then punched by a punch press to form green sheets 101
measuring 120 mm.times.120 mm for making the heater substrates 11
and a green sheet 102 measuring 120 mm.times.120 mm for making the
covering substrate 12. The through holes 71 to 74 are formed in the
green sheets 101 and 102.
The making of the green sheets 101 and 102 may alternatively be
achieved with the extrusion molding.
A conductive paste whose main constituent is metal such as W or Mo
is prepared and coated on surfaces of the green sheets 101 to form
heater-patterned layers 2, as shown in FIG. 7(a), and inner walls
of the through holes 71 to 74 using printing techniques. The
heater-patterned layers extend parallel to each other.
On a surface of each of the green sheets 101 opposite to the
heater-patterned layers 2, a conductive paste is coated to form the
metallic terminals 3 in line using printing techniques. The
conductive paste is made of a main constituent of metal containing
70 Wt % or more of W and a remaining content of Mo, but may be
identical with that used in forming the heater-patterned layers
2.
The two green sheets 101 are arranged so that the heater-patterned
layers 2 may face each other. Subsequently, the green sheet 102 is
interposed between the green sheets 101 to form a three-layer
laminate. The three-layer laminate is baked at 1400 to 1600.degree.
C. in a reducing atmosphere of N.sub.2 and H.sub.2 gasses to make
an intermediate.
The outer leads 4 are, as shown in FIG. 7(b), soldered to the
metallic terminals 3, respectively. The soldering is achieved by
placing solder and the outer leads 4 on the metallic terminals 3
and heating them at 1000 to 1200.degree. C. to form the bonding
layers 5.
Each of the outer leads 4 may be made either of a round bar, as
shown in FIG. 7(b), or of a square bar, as shown in FIGS. 8(b) and
8(b).
The overall surface of each of the bonding layers 5 is, as clearly
shown in FIG. 5(b), covered with an Ni-plated layer 6.
The intermediate is, as shown in FIG. 7(c), cut into several
pieces, i.e., units of the ceramic heaters 1.
Finally, an end of each of the ceramic heaters 1 opposite to the
outer leads 4 is rounded using a grinding machine.
Note that after the three-layer laminate is braked, the
intermediate is tested for heater performance.
Each of the bonding layers 5 may contain 40 to 98 Wt % of Cu and 2
to 20 Wt % of Ni. The metallic terminals 3, as described above,
contains W, thus resulting in improved wettability between the
bonding layers 5 and the metallic terminals 3, which eliminates the
need for the metallic terminals 3 to be plated with Ni in
conventional manufacturing processes.
When the content of Cu in the bonding layers 5 is small, less than
40 Wt % and when the leads 4 do not contain Ni, it will cause no Ni
to be diffused from the leads 4 to the bonding layers 5, so that
the content of Ni in the bonding layers 5 will be smaller than that
when the content of Cu is more than 40 Wt %, which results in
lowered wettability of the bonding layers 5 to the metallic
terminals 3 and a decrease in strength of joints of the bonding
layers 5 and the metallic terminals 3.
When the content of Cu in the bonding layers 5 is greater than 98
Wt %, the content of Ni in the bonding layers 5 will be smaller
than that in the metallic terminals 3, thereby causing the
wettability of the bonding layers 5 to the metallic terminals 3 to
be lowered, which results in a decrease in strength of the joints
of the bonding layers 5 and the metallic terminals 3.
When the content of Ni in the bonding layers 5 is less than 2 Wt %,
it will cause the wettability of the bonding layers 5 to the
metallic terminals 3 to be lowered, resulting in a decrease in
strength of the joints of the bonding layers 5 and the metallic
terminals 3. Alternatively, when the content of Ni in the bonding
layers 5 greater than 20 Wt %, it will cause a W--Ni intermetallic
compound to be precipitated during manufacture, resulting in a
decrease in strength of joints of the bonding layers 5 and the
metallic terminals 3.
The metallic terminals 3 contain, as described above, 70 Wt % of W
or more (including 100 Wt % of W) and thus have good conformability
to a ceramic particularly containing alumina (i.e., the square rod
10 of the ceramic heater 1) and good heat resistance. When the
content of W is less than 70 Wt %, it may result in decreases in
strength of a joint of the metallic terminals 3 and the square rod
10 and heat resistance.
The bonding layers 5 may contain 60 Wt % of Au or less for avoiding
precipitation of a W--Ni intermetallic compound to increase the
strength to join the leads 4 to the metallic terminals 3. When the
content of Au in the bonding layers 5 is more than 60 Wt %, the
content of Cu will be decreased. Thus, when the leads 4 do not
contain Ni, it will cause no Ni to be diffused from the leads 4 to
the bonding layers 5, so that the content of Ni in the bonding
layers 5 will be smaller than that when the content of Au is less
than 60 Wt %, which results in lowered wettability of the bonding
layers 5 to the metallic terminals 3 and a decrease in strength of
joints of the bonding layers 5 to the metallic terminals 3.
Specifically, when the content of Au is, as shown in FIG. 9, 60 to
90 Wt %, the hardness of the solder forming the bonding layers 5
becomes too high, thus resulting in a decrease in durability
against cyclic changes in ambient temperature. When the content of
Au is greater than 90 Wt %, the hardness of the solder is lower,
but manufacturing costs will increase.
A major surface of each of the metallic terminals 3 to which the
leads 4 are to be joined through the bonding layer 5 may be plated
with Ni. The thickness of the Ni-plated layer is 3 or less .mu.m.
The formation of the Ni-plated layer improves the wettability of
the bonding layer 5, thereby decreasing the welded angle which the
outer surface of each side end of the bonding layer 5 makes with
the metallic terminal 3, resulting in a decrease in thermal stress
contributing to cracks. When the thickness of the Ni-plated layer
is more than 3 .mu.m, a metallic alloy will be produced between the
Ni-plated layer and the metallic terminal 3 which decreases the
strength to join the bonding layer 5 and the metallic terminal
3.
The laminate produced in the process shown in FIG. 7(a) may consist
only of the single green sheet 101 and the green sheet 102. In this
case, the metallic terminals 3 are also formed on a surface of the
green sheet 102 opposite to a surface covering the heater-patterned
layers 2 of the green sheet 101.
As can be seen from the above discussion, the metallic terminals 3
and the outer leads 4 are disposed on the surfaces 17 and 18 of the
square rod 10 opposed in a direction of lamination of the
substrates 11 and 12, thereby allowing the joining process wherein
the outer leads 4 are joined to the metallic terminals 3,
respectively, to be performed before the intermediate is cut into
units of the ceramic heaters 1 in the course of manufacture. This
will result in great rationalization of the manufacturing
processes.
In addition, the performance test may be, as described above,
performed before the intermediate is cut into unit of the ceramic
heaters 1, thus resulting in rationalization of procedure of the
test.
The metallic terminals 3 and the bonding layers 5 are, as described
above, arranged on the surfaces 17 and 18 of the square rod 10 out
of alignment of side ends with each other, thus avoiding
concentration of stress on the side edges 171, 181, 31, and 51,
which will result in improved durability of the ceramic heater
1.
One of the metallic terminals 3 of the ceramic heater 1 may be
connected directly to a connector leading to, for example, a ground
terminal without use of the outer lead 4. In this case, the single
outer lead 4 may be joined to either of the metallic terminals
3.
FIG. 10 shows the second embodiment of the manufacturing processes
of the ceramic heater 1.
Before the three-layer laminate of the green sheets 101 and 102 is
braked, cutting notches or grooves 7 are machined in upper and
lower surfaces of the three-layer laminate which extend parallel
between adjacent two of the metallic terminals 3 for facilitating
ease of cutting the three-layer laminate into units of the ceramic
heaters 1 after being baked.
The formation of the cutting grooves 7 is achieved by grooving the
upper and lower surfaces of the three-layer laminate to a depth
less than half a thickness of the laminate using a cutting
machine.
Other manufacturing processes are identical with those of the first
embodiment, and explanation thereof in detail will be omitted
here.
Ten samples of the ceramic heater 1 made in the manufacturing
processes of the first embodiment were tested for the surface
roughness of the metallic terminals 3 which may be thought of as
one of factors of improvement of durability. The measurement of the
surface roughness was accomplished, as shown in FIG. 11(a), by
scanning the surface of the metallic terminal 3 of each sample over
0.8 mm in a direction, as indicated by S in FIG. 11(a). For
comparison, the same tests were performed, as shown in FIG. 11(b),
for ten conventional ceramic heaters identical with the one shown
in FIGS. 1(a) and 1(b). The results of the tests are shown in table
1 below.
TABLE 1 Sample Prior art Invention No. (.mu.m) (.mu.m) 1 3.642
1.481 2 3.932 1.098 3 2.47 1.018 4 3.782 0.978 5 3.146 1.294 6
2.858 1.893 7 3.431 1.149 8 3.278 1.19 9 2.685 1.435 10 2.891 1.215
Average 3.212 1.275
The table 1 shows that the surface roughness (Rz) of the metallic
terminals 3 of the ceramic heater 1 is greatly improved as compared
with the conventional ceramic heaters. The improvement of the
surface roughness of the metallic terminals will facilitate flow of
solder on the surfaces of the metallic terminals 3 when the outer
leads 4 are joined to the metallic terminals 3, thereby increasing
an area of the bonding layers 5, which results in improvement of
initial strength to join the outer leads 4 to the metallic
terminals 3 and a decrease in thermal stress acting on the joints
produced by cyclic temperature changes, thus improving the
durability of the ceramic heater 1.
FIG. 12 shows an oxygen sensor 8 in which the ceramic heater 1 is
built.
The oxygen sensor 8 is used in an automotive internal combustion
engine control system and includes a gas sensing element 81 with a
gas-exposed portion 811 exposed to the gas to be measured.
The gas sensing element 81 is of a cup-shape having formed therein
an inner chamber 810. Within the inner chamber 180, the ceramic
heater 1 is disposed for heating the gas sensing element 81.
On outer and inner surfaces of the gas sensing element 81, minus
and plus holders 86 and 87 are installed which have sensor signal
output terminals 869 and 879, respectively. The pulse holder 87
includes, as shown in FIGS. 13(b) and 14(a) to 14(c), a heater
holding portion 871 for holding the ceramic heater 1 and a sensor
contact 873 for making contact with the inner surface of the gas
sensing element 81. The sensor signal output terminal 879 extends
from an end of the sensor contact 873. The heater holding portion
871 and the sensor contact 873 have formed therein slits 877 and
878 to define C-shape in section so that they may be elastically
deformable to have spring properties. The slits 877 and 878 extend
in a lengthwise direction of the pulse holder 87 and are shifted
approximately 90.degree. away from each other.
The heater holding portion 871 and the sensor contact 873 are
joined through a frusto-conical connector 872. The connector 872
has formed therein an L-shaped slit which connects the slits 877
and 878. The heater holding portion 871 and the sensor contact 873
are eccentric so that the ceramic heater 1 may be coaxial with the
gas sensing element 81 when the plus holder 87 is fitted in the gas
sensing element 81.
The slit 878 formed in the sensor contact 873 is, as can be seen in
FIG. 13(b), diametrically opposed to the sensor signal output
terminal 879 and thus is located at an angular interval of
90.degree. away from the slit 877 formed in the heater holding
portion 871.
The sensor contact 873 has formed on the end thereof a plurality of
claws 874 which engage an upper end of the gas sensing element 81
for orientation to the gas sensing element 81.
The sensor contact 873 has an outer diameter slightly greater than
an inner diameter of the gas sensing element 81 so that the sensor
contact 873 may be installed elastically within the gas sensing
element 81 by a press fit. The heater holding portion 871 has an
inner diameter slightly smaller than a maximum outer diameter of
the ceramic heater 1 for establishing tight engagement with the
ceramic heater 1 when fitted in the heater holding portion 871.
The minus holder 86, as clearly shown in FIG. 13(a), has formed
therein a slit to have spring properties like the plus holder 87.
In order to enhance the spring properties, the plus holder 87 and
the minus holder 86 are both made of a heat resisting spring
steel.
FIGS. 15(a), 15(b), and 16 show the plus holder 87 in which the
ceramic heater 1 is fitted.
As clearly shown in FIG. 16, the ceramic heater 1 is disposed in
the plus holder 87 with one of the surfaces on which the outer
leads 4 are installed facing the slit 877 so that the outer leads 4
may be both located 90.degree. apart from the sensor signal output
terminal 879.
FIG. 17 shows the plus holder 87 holding therein the ceramic holder
1 fitted in the gas sensing element 81 and the minus holder 86
installed on the outer surface of the gas sensing element 81. The
sensor signal output terminal 869 of the minus holder 86 is located
approximately 180.degree. away from the sensor signal output
terminal 879 of the plus holder 87. The sensor signal output
terminals 869 and 879 are, therefore, arranged at angular intervals
90.degree. away from the outer leads 4, respectively.
The gas sensing element 81 has, as shown in FIG. 12, a reference
gas chamber 812 formed in the inner chamber 810 and defines a gas
chamber 813 between itself and a protective cover assembly 82. An
outer electrode 815 and an inner electrode 814 both made of
platinum are installed on the gas-exposed portion 811 and the inner
surface of the gas sensing element 81 in connection with the minus
holder 815 and the plus holder 87, respectively.
The sensor signal output terminals 869 and 879 of the holders 86
and 87 and the leads 4 of the ceramic heater 1 are electrically
connected to four leads 891 to 893, respectively, through
connectors 995 and 896. The connectors 995 and 895 are disposed in
an insulator 85 at regular intervals of 90.degree. for avoiding
interference with each other.
The gas sensing element 81 is installed in a sensor mount 84 which
is used in mounting the oxygen sensor 8 in an exhaust pipe of an
automotive engine. The protective cover assembly 82 is mounted on
an end of the sensor mount 84 to cover the gas sensing element 81.
A dust cover 83 is mounted on the sensor mount 84.
The sensor mount 84 has a cylindrical wall which extends upward
from the flange thereof and in which an insulator 881, a talc 882,
and a ring spacer 883 are disposed to retain the gas sensing
element in the sensor mount 84. An end 841 of the cylindrical wall
of the sensor mount 84 is crimped inward to elastically press the
ring spacer 883 downward, as viewed in FIG. 12. A float packing 884
is interposed between an inner wall of the sensor mount 84 and an
outer wall of the gas sensing element 81 to seal the gas chamber
813 hermetically.
The sensor mount 84 has formed in the end 842 thereof an annular
groove 843 to form an outer skirt 844 and an inner skirt 845. The
protective cover assembly 82 consists of an outer cover 821 and an
inner cover 822 both made of a cup-shaped member. The outer and
inner covers 821 and 822 have flanges 828 and 829 which are
retained in the groove 843 of the sensor mount 84 by crimping the
outer skirt 844 inward. The outer and inner covers 821 and 822 have
formed in side walls thereof a plurality of holes through which a
gas to be measured passes to enter the gas chamber 813.
The dust cover 83, as shown in FIG. 12, consists of a
small-diameter cylinder 831, a large-diameter cylinder 832, and a
shoulder portion 833 connecting the cylinders 831 and 832. The dust
cover 83 is, as described above, welded at a circumferential
portion 834 thereof to a boss of the sensor mount 84 and retains
therein the insulator 85.
A cylindrical cover 839 is mounted on the periphery of the
small-diameter cylinder 831 of the dust cover 83 by crimping. A
water-repellent filter 857 is installed between the cylindrical
cover 839 and the small-diameter cylinder 831. The cover 839 and
the dust cover 83 have formed therein first air vents 858 and
second air vents 859, respectively, which communicate with the
reference gas chamber 812 formed in the gas sensing element 81 to
fill the reference gas chamber 812 with air.
A heat-resisting rubber bush 895 is mounted in the end of the
small-diameter cylinder 831 of the dust cover 83 to retain the
leads 891 to 893 at angular intervals of 90.degree..
The insulator 85 consists of a sleeve 851 in which the leads 891 to
893 are disposed and a flange 852 greater in diameter than the
sleeve 851. The small-diameter cylinder 831 of the dust cover 83
has the inner diameter greater than the outer diameter of the
sleeve 851 of the insulator 85 and smaller than the outer diameter
of the flange 852. The large-diameter cylinder 832 of the dust
cover 83 has the inner diameter greater than the outer diameter of
the flange 852 of the insulator 85.
The insulator 85 is retained in the large-diameter cylinder 832 of
the dust cover 83 in engagement of an upper end of the flange 852
with the shoulder portion 833 of the dust cover 83 by a stop ring
899 press-fitted in the large-diameter cylinder 832.
The gas sensing element 81 produces the electromotive force as a
function of a difference in oxygen concentration between the air in
the reference gas chamber 812 and the gas in the gas chamber 813
and outputs a signal indicative thereof through the leads 891 and
892. The operation of the oxygen sensor 8 is well known in the art,
and explanation thereof in detail will be omitted here.
The operation and effects of this embodiment will be described
below.
The four connectors 896 and 995 are disposed in an insulator 85 at
regular intervals of 90.degree. for avoiding interference with each
other. The sensor signal output terminals 879 and 869 of the
holders 86 and 87 and the leads 4 of the ceramic heater 1 are,
therefore, located at regular intervals of 90.degree. away from
each other.
The sensor signal output terminal 879 installed on the sensor
contact 873 of the plus holder 87 is, as described above, located
approximately 90.degree. away from the slit 877 formed in the
heater holding portion 871, thereby allowing the ceramic heater 1
to be, as shown in FIGS. 16 and 17, fitted firmly in the heater
holding portion 871 of the plus holder 87 so that the leads 4 of
the ceramic heater 1 may be located at angular intervals of
90.degree. away from the sensor signal output terminal 879.
For comparison with this embodiment, a plus holder 97 used in
conventional oxygen sensors is shown in FIGS. 18(a) and 18(b). The
plus holder 97 has a slit 977 formed in a heater holding portion
971 at an angular interval of 180.degree. away from a sensor signal
output terminal 979. The slit 977 is located at the same angular
position as that of a slit 978 formed in a sensor contact 973 of
the plus holder 97. Arranging the leads 4 of the ceramic heater 1
90.degree. apart from the sensor signal output terminal 979
requires, as shown in FIG. 18(a), retaining side walls of the
ceramic heater 1 between vertical edges 999 and an opposite inner
wall of the heater holding portion 971 defining the slit 977, thus
resulting in instability of installation of the ceramic heater
1.
The stable installation of the ceramic heater 1 in the plus holder
97 requires, as shown in FIG. 18(b), retaining the side walls of
the ceramic heater 1 between opposite portions of the inner wall of
the plus holder 97 located 90.degree. apart from the slit 977. In
this case, the leads 4 are oriented in alignment with the sensor
signal output terminal 979, so that they are twisted undesirably
when connected to the connectors 896 and 995.
The structure of this embodiment allows, as described above, the
leads 4 of the ceramic heater 1 to be located 90.degree. apart from
the sensor signal output terminal 879 without compromising the
installation of the ceramic heater 1 in the plus holder 87.
The positional relation between the sensor signal output terminal
879 of the sensor contact 973 and the slit 877 of the heater
holding portion of the plus holder 87 is not limited to 90.degree.,
but may be within an angular range of 90.degree..+-.20.degree..
This also achieves firm installation of the ceramic heater 1 in the
plus holder 87 without interfering the connectors 896 and 995 with
each other.
The inventors of this application analyzed the relation between the
durability of the ceramic heater 1 and a welded angle which the
outer surface of each side end of the bonding layer 5 makes with
the metallic plate 3. The analysis was made by preparing samples
whose welded angles .gamma., as shown in FIG. 19, are 25.degree. to
60.degree. and performing a temperature cycle test a hundred times
in which each sample was subjected to intense heat at 450.degree.
C. for four minutes and then left at room temperature for four
minutes. After the hundred temperature cycle tests, each metallic
terminals 3 was checked for cracks, and the strength of a joint of
the bonding layer 5 and the metallic terminal 3 was measured. The
measurement of the strength was performed in tensile tests. The
results of the tests are shown in table 2 below.
TABLE 2 Welded Joint Angle Strength .gamma. Cracks (kgf) Evaluation
60.degree. many 1 or less X 50.degree. many 1 or less X 40.degree.
few 3 .DELTA. 30.degree. few 4 .smallcircle. 20.degree. few 4.5
.smallcircle.
where .smallcircle. indicates excellent durability, .DELTA.
indicates allowable durability, and X indicates lack of
durability.
The table 2 shows that the ceramic heater 1 has high durability
when the welded angle .gamma. is 40.degree. or less.
While the present invention has been disclosed in terms of the
preferred embodiments in order to facilitate better understanding
thereof, it should be appreciated that the invention can be
embodied in various ways without departing from the principle of
the invention. Therefore, the invention should be understood to
include all possible embodiments and modifications to the shown
embodiments which can be embodied without departing from the
principle of the invention as set forth in the appended claims.
* * * * *